In proton exchange membrane (PEM) fuel cells, hydrogen is supplied to the anode and oxygen is supplied as the oxidant to the cathode. The overall reaction consumes hydrogen at the anode and produces water (product water) at the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive non-electrically conductive solid polymer electrolyte membrane having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which 1) serve as current collectors for the anode and cathode, and 2) contain appropriate channels and/or openings for distributing the fuel cells gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context.
Efficient operation of the fuel cell depends on the ability to effectively disperse reactant gases at catalytic sites of the electrode where reaction occurs. In addition, effective removal of reaction products is required so as to not inhibit the flow of fresh reactants to the catalytic site. Therefore, it is desirable to improve the mobility of reactant and product species to and from the MEA where reaction occurs.
Gas diffusion media play an important role in PEM fuel cells. Generally disposed between catalytic electrodes and flow field channels in the fuel cell, they provide reactant and product permeability, electronic conductivity, and heat conductivity, as well as mechanical strength needed for proper functioning of the fuel cell. One of its most important functions is water management. For example, the diffusion media prevents flooding by wicking product water away from the catalyst layer while maintaining reactant gas flow from the bipolar plate through to the catalyst layer. The gas diffusion media are generally constructed of carbon fiber containing materials.
Although carbon fibers are themselves relatively hydrophobic, it is usually desirable to increase the hydrophobicity or to at least treat the carbon fiber with a coating to give a more stable hydrophobic coating. Adding a hydrophobic agent such as polytetrafluoroethylene (PTFE) to the carbon fiber diffusion media is a common process for increasing and/or stabilizing the hydrophobicity. This process is normally done by dipping carbon fiber papers into a solution that contains PTFE particles and surfactants.
Even though coating the diffusion media with PTFE generally improves cell performance, existing processes for preparing the coated diffusion media results in diffusion media having inconsistent results. Such inconsistent results may manifest themselves for example, in fuel cells wherein the voltage of the cell is undesirably dependent on the amount of gas flow.
It would therefore be desirable to provide fuel cells having improved consistent performance. It has been surprisingly discovered that the performance of fuel cells can be noticeably improved by providing the fuel cells with water management through the use of specially prepared gas diffusion media.